Re: [Patch v1] iommu: arm-smmu: disable large page mappings for Nvidia arm-smmu

[Robin Murphy <robin.murphy@arm.com>]

On 2022-04-17 10:04, Ashish Mhetre wrote:
> Tegra194 and Tegra234 SoCs have the erratum that causes walk cache
> entries to not be invalidated correctly. The problem is that the walk
> cache index generated for IOVA is not same across translation and
> invalidation requests. This is leading to page faults when PMD entry is
> released during unmap and populated with new PTE table during subsequent
> map request. Disabling large page mappings avoids the release of PMD
> entry and avoid translations seeing stale PMD entry in walk cache.
> Fix this by limiting the page mappings to PAGE_SIZE for Tegra194 and
> Tegra234 devices. This is recommended fix from Tegra hardware design
> team.

Is this related to any of the several known MMU-500 invalidation errata, 
or is it definitely specific to something NVIDIA have done with their 
integration?

> Co-developed-by: Pritesh Raithatha <praithatha@nvidia.com>
> Signed-off-by: Pritesh Raithatha <praithatha@nvidia.com>
> Signed-off-by: Ashish Mhetre <amhetre@nvidia.com>
> ---
>   drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c | 23 ++++++++++++++++++++
>   drivers/iommu/arm/arm-smmu/arm-smmu.c        |  3 +++
>   drivers/iommu/arm/arm-smmu/arm-smmu.h        |  1 +
>   3 files changed, 27 insertions(+)
> 
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c b/drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c
> index 01e9b50b10a1..b7a3d06da2f4 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-nvidia.c
> @@ -258,6 +258,27 @@ static void nvidia_smmu_probe_finalize(struct arm_smmu_device *smmu, struct devi
>   			dev_name(dev), err);
>   }
>   
> +static void nvidia_smmu_cfg_pgsize_bitmap(struct arm_smmu_device *smmu)
> +{
> +	const struct device_node *np = smmu->dev->of_node;
> +
> +	/*
> +	 * Tegra194 and Tegra234 SoCs have the erratum that causes walk cache
> +	 * entries to not be invalidated correctly. The problem is that the walk
> +	 * cache index generated for IOVA is not same across translation and
> +	 * invalidation requests. This is leading to page faults when PMD entry
> +	 * is released during unmap and populated with new PTE table during
> +	 * subsequent map request. Disabling large page mappings avoids the
> +	 * release of PMD entry and avoid translations seeing stale PMD entry in
> +	 * walk cache.
> +	 * Fix this by limiting the page mappings to PAGE_SIZE on Tegra194 and
> +	 * Tegra234.
> +	 */
> +	if (of_device_is_compatible(np, "nvidia,tegra234-smmu") ||
> +	    of_device_is_compatible(np, "nvidia,tegra194-smmu"))
> +		smmu->pgsize_bitmap = PAGE_SIZE;
> +}
> +
>   static const struct arm_smmu_impl nvidia_smmu_impl = {
>   	.read_reg = nvidia_smmu_read_reg,
>   	.write_reg = nvidia_smmu_write_reg,
> @@ -268,10 +289,12 @@ static const struct arm_smmu_impl nvidia_smmu_impl = {
>   	.global_fault = nvidia_smmu_global_fault,
>   	.context_fault = nvidia_smmu_context_fault,
>   	.probe_finalize = nvidia_smmu_probe_finalize,
> +	.cfg_pgsize_bitmap = nvidia_smmu_cfg_pgsize_bitmap,
>   };
>   
>   static const struct arm_smmu_impl nvidia_smmu_single_impl = {
>   	.probe_finalize = nvidia_smmu_probe_finalize,
> +	.cfg_pgsize_bitmap = nvidia_smmu_cfg_pgsize_bitmap,
>   };
>   
>   struct arm_smmu_device *nvidia_smmu_impl_init(struct arm_smmu_device *smmu)
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> index 568cce590ccc..3692a19a588a 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> @@ -1872,6 +1872,9 @@ static int arm_smmu_device_cfg_probe(struct arm_smmu_device *smmu)
>   	if (smmu->features & ARM_SMMU_FEAT_FMT_AARCH64_64K)
>   		smmu->pgsize_bitmap |= SZ_64K | SZ_512M;
>   
> +	if (smmu->impl && smmu->impl->cfg_pgsize_bitmap)
> +		smmu->impl->cfg_pgsize_bitmap(smmu);

I'm not the biggest fan of adding a super-specific hook for this, when 
it seems like it could just as easily be handled in the init_context 
hook, which is where it is precisely for the purpose of mangling the 
pgtable_cfg to influence io-pgtable's behaviour.

Thanks,
Robin.

> +
>   	if (arm_smmu_ops.pgsize_bitmap == -1UL)
>   		arm_smmu_ops.pgsize_bitmap = smmu->pgsize_bitmap;
>   	else
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.h b/drivers/iommu/arm/arm-smmu/arm-smmu.h
> index 2b9b42fb6f30..5d9b03024969 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.h
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.h
> @@ -442,6 +442,7 @@ struct arm_smmu_impl {
>   	void (*write_s2cr)(struct arm_smmu_device *smmu, int idx);
>   	void (*write_sctlr)(struct arm_smmu_device *smmu, int idx, u32 reg);
>   	void (*probe_finalize)(struct arm_smmu_device *smmu, struct device *dev);
> +	void (*cfg_pgsize_bitmap)(struct arm_smmu_device *smmu);
>   };
>   
>   #define INVALID_SMENDX			-1
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